I reread the arc and several elements intrigue me. Hiroto's mother asks if her husband will be absent again, and Hiroto's sister hesitates before answering, claiming he's busy with work. (Ch. 14)
Chika (Hiroto's sister) seems to be living with their mother. It's strange for a woman with a family life. I might just not understand the culture. But I wonder if she's on the run. It would explain why her husband is never there since she's avoiding him.
Thirdly, Chika's facial expression shows a lot of concern when she receives messages or avoids calls. In chapter 15, Hiroto remarks to her, 'you've always been worried about others the most... even when you yourself are in trouble,' and again, a moment of worry from Chika before she decides to talk to her brother and he might be able to help her.
Then we have the famous message asking if it's still the same with her husband and that the person Chika is talking to is ready to help her.
Fourthly, during chapter 16, Chika once again shows a worried face when seeing her phone and claims this time it's a call from her part-time job, she wants to go to a quieter place to call this person back, and she disappears.
With all these elements, the theory of cheating is not ruled out and is even highly likely. However, Chika shows a lot of concern and fear, which leads me to think that she may be in a toxic relationship with her husband, perhaps experiencing domestic violence, that she left her family home, is hiding there waiting to find a solution, and during the festival it's precisely her husband who calls her.
If it were her lover, for example, I think she would have been more excited to receive his calls or messages, you know what I mean.
It's a shame that the chapters are monthly and so 'short'. I don't think Toko will put the baby in danger, on the other hand, what she's capable of is manipulation. She could either blackmail if Chika cheats on her husband, or help her if she's in a toxic relationship to gain her sympathy.